Composite polyester films with barrier properties

ABSTRACT

The invention concerns composite polyester films with good barrier properties relative to gases. More precisely, it concerns a biaxially drawn film, based on polyester and having improved barrier properties relative to gases, characterised in that it comprises a base polyester film of 5 to 50 μm, coated on one of its two surfaces with a deposit containing polyvinyl alcohol and a water dispersible copolyester with sulphonyloxy units, said composite film having permeability to oxygen measured at 23° C. under 50% relative humidity, not more than 5 cm 3 /m 2 /24 h. Said composite films have, besides being transparent and having barrier properties, excellent resistance to mechanical stresses, such as in particular crumpling.

The present invention is concerned with composite films based onpolyester and having good barrier properties against gases.

Polyester films, in particular polyethylene terephthalate, are largelyused for packaging, owing to their numerous advantages, such as theirmechanical properties, their transparency, their lack of toxicity, andbecause they have no taste or odour.

However, their properties as barriers for gases can limit their use inapplications requiring a high level of protection for the packageditems, against the action of outside gases, in particular against theaction of the oxygen of air, or conversely requiring that the gascomposition within the packaging remains constant.

In order to overcome this drawback, it was proposed in GB-A-1 126 952patent to deposit on a polymer film a solution of polyvinyl alcohol, inorder to create a layer of polyvinyl alcohol, the latter having goodgas-barrier properties. In order to enable a good adhesion of thepolyvinyl alcohol layer on films of acetate cellulose, polycarbonate orpolyethylene terephthalate, an intermediate layer of a polyurethaneadhesive is provided between the film base and the polyvinyl alcohollayer.

Patent EP-A-0 254 468 also describes a composite film comprising a filmbase of a synthetic thermoplastic polymer, such as a polyamide, apolyethylene, a polypropylene or a polyester having two coatings on thesame face of a film base; the first coating being adjacent to the filmbase and being comprised of a urethane primer applied in a solvent,which enables, when dry, a dispersion in an aqueous solution of apolyvinyl alcohol to wet the primer coating, the second coating beingplaced on the dried surface of the first coating and including amaterial based on a polyvinyl alcohol acting as a barrier to gases.

The present invention is concerned with polyester films having goodbarrier properties to gases, owing to a coating including a polyvinylalcohol placed directly on the surface of the polyester film without thepresence of an intermediate adhesive layer, while providing a goodadhesion between the polyester film base and the coating.

More precisely, it is concerned with a composite film stretchedbiaxially, based on a polyester and having improved barrier propertiesto gases, characterised in that it includes a polyester film base of 6μm to 50 μm, coated on one of its two faces with a layer including apolyvinyl alcohol which has an average degree of polymerisation innumber equal to or in excess of 350 and which is hydrolysed to a degreeof hydrolysis of 95% or more and a water-dispersible copolyester withsulphonyloxy residues, said composite film exhibiting a permeability tooxygen measured at 23° C. under a relative humidity of 50%, equal to orlesser than 5 cm³/m²/24 h.

A degree of hydrolysis (or degree of saponification) of 95% of thepolyvinyl alcohol means that said polyvinyl alcohol includes 95% vinylalcohol residues in its formula. The terms <<degree of hydrolysis>>,<<degree of saponification>> or <<content of polyvinyl alcoholresidues>> will therefore be used interchangeably in the presentspecification.

The polyester forming the film base can be selected from thosepolyesters which are normally used for obtaining semi-crystallinebi-oriented films. These are film-forming linear polyesters, whichcrystallise when oriented and which are obtained in the usual mannerfrom one or several aromatic dicarboxylic acids or their derivatives(for example, esters of lower aliphatic alcohols or halogenides) andfrom one or several aliphatic glycols. As examples of aromatic adds, onecan mention phthalic acid, terephthalic acid, isophthalic acid,naphthalene 2,5-dicarboxylic acid and naphthalene 2,6-dicarboxylic acid.These acids can be associated to a minor amount of one or severalaliphatic or cycloaliphatic dicarboxylic acids, such as adipic acid,azelaic acid and hexahydroterephthalic acid. As non limiting examples ofaliphatic diols, one can mention ethylene glycol, 1,3-propanediol and1,4-butanediol.

These diols can be associated with a minor amount of one or severalaliphatic diols with a higher carbon content (neopentylglycol forexample) or with cycloaliphatic diols (cyclohexanedimethanol).Preferably, the film-forming crystallisable polyesters arepolyterephthalates or polynaphthalene dicarboxylates of alkylene diolsand, in particular polyterephthalate of ethylene glycol (PET), of1,4-butanediol or copolyesters thereof including at least 80% in molesethylene glycol terephthalate residues. Advantageously, the polyester isa polyterephthalate of ethylene glycol having an intrinsic viscositymeasured at 25° C. in ortho-chlorophenol comprised between 0.6 and 0.75dl/g.

The polyester forming the film base must be selected in such a mannerthat its temperature at the beginning of the melting be higher than thetemperature at which the bi-stretched film comprising the layer of thepolyvinyl alcohol and of the water-dispersible copolyester is heatedduring its preparation.

When the thickness of the film including the polyvinyl alcohol and awater-soluble copolyester with sulphonyloxy residues is lesser than 0.6μm, the average roughness Rz of the film base (such as defined instandard DIN 4768) is lesser than or equal to 0.40 μm on the face of thefilm carrying the layer of the polyvinyl alcohol and of thewater-dispersible copolyester and this face exhibits, on the average,not more than 20 peaks having a height equal to or in excess of 1micrometre and not more than 150 peaks having a height comprised between0.4 and 1 micrometre, per square millimeter.

The peak height distribution mentioned above for defining the surfacetopography of the polyester film base, can be determined, in a knownmanner, and in particular through visual inspection with aninterferential microscope which makes it possible to count the number ofinterference rings for a light having a known wavelength. Theinterferometers most frequently used are the NOMARSKI, the MIRAU and theMICHELSON interferometers.

Concerning the industrial implementation of these processes for thepreparation of films according to the invention, in which the speed ofthe machine in which the films are processed is generally in excess of100 meters per minute, it is particularly preferable that the face ofthe film base carrying the layer of polyvinyl alcohol and of thewater-dispersible copolyester have no more than an average of 20 peakshaving a height equal to or in excess of 1 micrometre and no more than100 peaks having a height comprised between 0.4 and 1 micrometre, persquare millimeter.

The water-dispersible copolyesters with sulphonyloxy residues combinedwith the polyvinyl alcohol in the coating are copolyester derived fromat least one aromatic dicarboxylic acid and from at least one aliphaticdiol, and carrying a plurality of sulphonyloxy groups of the generalformula (I):

—(SO₃—)_(n)M  (I)

in which n is equal to 1 or 2, M is a hydrogen atom, an alkali metal, anearth-alkali metal, a cationic ammonium or a cationic quaternaryammonium.

The term <<water-dispersible copolyester>> designates in the presentapplication copolyesters which are soluble in water or copolyestersforming stable homogeneous dispersions.

The water-dispersible copolymers carrying sulphonyloxy residues arealready known, in particular from patent FR 1 602 002 and from patentEP-A-0 540 374, to which one can refer for more detailed information ontheir preparation and their composition. They are obtained through thepolycondensation of one or of several aromatic dicarboxylic acids withone or several aliphatic diols and at least one difunctional compoundincluding at least one sulphonyloxy group of formula (I). Forconvenience sake, in the remainder of the specification, the term<(sulphonyloxy group>>designates both hydroxysulphonyl groups and alkalimetal salts, alkaline-earth metal salts or ammonium salts derivedtherefrom.

Amongst the aromatic dicarboxylic acids used for preparing thewater-dispersible copolyesters, one can mention, by way of example,terephthalic acid, isophthalic acid, orthophthalic acid and naphthalene1,4-dicarboxylic acid. These acids can be used alone or as mixtures.Amongst the above-mentioned acids, one can use preferably, for thepreparation of water-dispersible copolyesters entering into thecomposition of the films of the invention, terephthalic acid orisophthalic acid alone, mixtures thereof, or their mixtures with otheraromatic dicarboxylic acids. Mixtures of terephthalic acid with one orseveral other aromatic dicarboxylic acids and specially with isophthalicacid, are particularly well-suited. In this case, the amount ofterephthalic add, expressed in moles, can vary between 20 and 99% of thetotal number of moles of non sulphonated diacids and, preferably,between and 95%.

Concerning the preparation of the water-dispersible copolyester,aliphatic dicarboxylic acids including 3 to 15 atoms of carbon can beassociated with the aromatic diacids. More particularly, a portion orthe totality of the aromatic dicarboxylic acid used with theterephthalic acid, such as isophthalic acid, can be replaced by one orseveral aliphatic acids such as adipic acid, glutaric acid, succinicacid, suberic add, sebacic acid and dodecanoic acid.

As examples of diols entering into the composition of thewater-dispersible copolyesters with sulphonyloxy groups, one can mentionethylene glycol, 1 ,4-butanediol, 1,5-butanediol, 1,3-propanediol,1,2-propanediol, 2,2-dimethyl-1,3-pronanediol (or neopentylglycol),1,5-pentanediol, 1,6-hexanediol, diethylene glycol, triethylene glycol,tetraethylene glycol and cyclohexane dimethanol. Ethylene glycol and itsoligomers are particularly suitable. They can be used either alone or inmixtures thereof and/or with other diols. Preferably, residues derivedfrom ethylene glycol and from its oligomers are present in thewater-dispersible copolyesters.

The sulphonyloxy groups of the water-ispersible copolyesters areintroduced via a difunctional compound carrying a sulphonyloxy group,capable of reacting with diols during the polycondensation. Examples ofsuch compounds are cited in the patent EP-A 0 540 374. Preferably, thesulphonyloxy residues are derived from salts of alkali metals ofaromatic dicarboxylic acids such as sulphoterephthalic acid,sulfoisophthalic acid, sulphophthalic acid, 4-sulphonaphthalene2,7-dicarboxylic acid, sulpho4,4′-bis(hydroxycarbonyl)diphenylsulphones,sulphodiphenyidicarboxylic acid, 0osulpho-4,4′-bis(hydroxycarbonyl)diphenylmethane,5-sulphophenoxyisophthalic acid.

Water-dispersible copolyesters including sodium5-oxysulphonylisophtalate residues are preferred.

In the water-dispersible copolyesters, the dicarboxylic acid residueswith sulphonyloxy groups represent 5-30 moles per 100 moles of thedicarboxylic acid residues and, preferably 8-15 moles per 100 moles.

The coating of the polyvinyl alcohol and of the water-dispersiblecopolyester exhibits a weight ratio of the polyvinyl alcohol to thewater-dispersible copolyester of 99/1 to 50/50. Preferably, this weightratio of the polyvinyl alcohol to the water-dispersible copolyester iscomprised between 97/3 and 80/20 and, even more preferably between 95/5and 85/15.

The other face of the composite films according to the invention, theback face thereof, must exhibit slippage characteristics sufficient forenabling an easy handling of the film, in particular when stretching thesame on guide rollers and when winding up of the same on uptake rollers.These slippage characteristics can be brought about in differentmanners.

One of the most usual methods consists in incorporating into thepolyester, before its transformation into a film, solid inert fillers.These fillers are generally mineral fillers, such as for example silica,titanium oxide, zirconium oxide, alumina, mixtures of silica andalumina, silicates, calcium carbonate and barium sulphate. These fillerscan also be comprised of particles of polymers.

The average diameter in volume of the fillers is generally comprisedbetween 1 and 10 micrometres and, preferably, between 1 and 5micrometres.

The filler content of the film is usually comprised between 0.02% and 1%weight relative to the weight of the polyester.

An interesting version of the invention consists in producing apolyester film base having surface properties, in particular roughnessproperties, which are different for each one of the two faces. Thisdiffering average roughness Rz can, for example, be higher than 0.15micrometres on the back face of the film and be equal to or lesser than0.40 micrometres on the face of the film base carrying the coating ofthe polyvinyl alcohol and of the water-dispersible copolyester.

Thus, the polyester film base can be formed of two layers havingdiffering surface properties, in particular differing roughnessproperties.

Such dissymetrical films can be obtained using a co-extrusion techniqueof 2 polyesters having differing filler contents and, possibly fillersof differing nature. Advantageously, the polyester used is the same forthe two co-extruded layers and the layer which is to receive the coatingof polyvinyl alcohol and of the dispersible copolyester has a lowerfiller content. The relative thickness of the 2 polyester layers formingthe polyester film base can vary broadly.

Generally, the layer which has the lower filler content and receivingcoating of the polyvinyl alcohol and of the water-dispersiblecopolyester has a thickness equal to or in excess of 0.5 μm and,preferably equal to or in excess of 1.0 μm.

Film bases of polyester exhibiting differing surface properties on eachone of the faces and obtained by other means known from prior art arenor excluded from the field of the invention.

One can thus confer, according to patent EP-A-0 378 954 good slippagecharacteristics to the back face of a film base by depositing on saidback face a modified polymer obtained through a radical polymerisationin an aqueous phase of at least one acrylic monomer and of a polyesterdispersible in water, derived from at least one aromatic dicarboxylicacid and from at least one aliphatic diol and including a plurality ofsulphonyloxy groups, in particular of sodium sulphonate groups.

Depending on the coating process used, the coating of the polyvinylalcohol and of the water-dispersible copolyester can have a thicknessequal to or lesser than 3 μm. This thickness can be, if desired, lesserthan 0.60 μm or even lesser than 0.20 μm, should one wish to facilitatethe recycling of the film according to the invention. In practicehowever, only on rare occasions will the thickness be lesser than 0.05μm.

An advantageous version of the composite films of the invention isconcerned with films which carry a layer of polyvinyl alcohol on top ofthe coating of the polyvinyl alcohol and of the water-dispersiblecopolyester. This additional layer makes it possible to further decreasethe permeability to gases of the composite films. By way of example, itcan generally have a thickness of 0.1 μm to 10 μm.

The invention is also concerned with composite films such as thosedescribed previously, which furthermore carry a printed layer or aprinted primer on one of their faces or a metal layer on the free faceof the coating of the polyvinyl alcohol and of the water-dispersiblecopolyester or on the additional layer of polyvinyl alcohol, whenprovided.

The printed layer can be deposited by known printing methods such asgravure printing, flexographic printing, or serigraphy. Preferably, inksolutions or aqueous dispersions will be used.

The metal layer which is deposited on the surface of the coating of thepolyvinyl alcohol and of the water-dispersible polyester which is not incontact with the polyester film or which is deposited on the free faceof the additional layer of the polyvinyl alcohol is made of a metalwhich can be deposited using different known techniques. The metal isselected most often from aluminium, copper, chromium, nickel and silver.For practical and economical reasons, the metal used is preferablyaluminium.

The thickness of the metal layer is generally between 0.01 μm and 0.06μm. This thickness is determined by the measure of the optical density(measurements carried out with an apparatus MACBETH-TD102). Thesethicknesses correspond to optical densities of 1 to 3.6.

On the free face of the polyester film base, one can also apply aheat-sealable layer which can be of the polyolefin type (such as:radical polyethylenes, linear polyethylenes, metallocene polyethylenes,polypropylenes, ethylene - vinyl acetate copolymers, ionomer resins)and/or copolyesters. These complex films can be obtained by laminationusing a single or a two component adhesive or by extrusion coating, byco-extrusion or by spread coating. This heat-sealable layer can functiononly as a sealable layer or have the double function of a sealable andof a peelable layer.

The complex films thus construed can be used for making packaging items,such as bags, membrane seals for trays, and secondary wrapping designedfor packaging products sensitive to oxidation. They are particularlywell adapted to packaging under a modified atmosphere. The presentinvention is also concerned with a process for the production of theabove-described films.

More particularly, it concerns a process for the production of compositefilms based on a polyester film which includes on one of its faces acoating of a polyvinyl alcohol and of a water-dispersible polyester,said process being characterised in that the polyester film base iscoated on one of its faces with an aqueous solution including apolyvinyl alcohol having at least 95% of vinyl alcohol residues, saidpolyvinyl alcohol having, in a 4% aqueous solution and at 20° C., aviscosity which is equal to or in excess of 4 mPa.s and awater-dispersible copolyester, the coating of the polyvinyl alcohol andof the water-dispersible polyester being in turn coated, if desired,with an aqueous solution of a polyvinyl alcohol, and in that said coatedfilm is subjected to a thermal treatment at a temperature equal to or inexcess of 170° C.

The thickness of the coating of the polyvinyl alcohol and of thewater-dispersible polyester is preferably lesser than 3 micrometres.

The thickness of the additional layer of polyvinyl alcohol is,preferably, from 0.1 to 10 micrometres.

When it is desired to produce a coating of a polyvinyl alcohol and of awater-dispersible ester which has a thickness lesser than 0.6micrometres, it is preferable that the face of the polyester film onwhich is applied the solution of the polyvinyl alcohol and of thewater-dispersible co-polyester to produce a coating exhibit an averagesurface roughness Rz lesser than or equal to 0.40 μm and include on theaverage not more than 20 peaks having a height equal to or in excess of1 micro-metre and not more than 150 peaks having a height comprisedbetween 0.4 and 1 micrometre per square millimeter.

The coating of the polyester film base by the aqueous solution of thepolyvinyl alcohol and of the water-dispersible copolyester can becarried out on-line or as a separate operation. When the coatingoperation is carried out on-line, the thickness of the coating is lesserand it is preferable to have a film surface topography which is morestrictly defined.

In such a case, when the process according to the invention isimplemented, for example in an industrial production using a high-speedequipment (generally running at more than 100 metres per minute), it isadvantageous that the coating by the aqueous solution of the polyvinylalcohol and of the water-dispersible copolyester be carried out on aface of the polyester film not carrying more than 20 peaks having aheight equal to or in excess of 1 micrometre and not more than 100 peakshaving a height comprised between 0.4 and 1 micrometre, per squaremillimeter.

When one carries out a coating operation as a separate step, thepolyester film surface can be rougher than in the case of the on-linecoating, without this however being a necessity.

Before the coating operation of the film base, the surface thereof isgenerally subjected to a physical treatment (such as a corona, flame orplasma treatment) designed for ensuring a good spreading of thepolyvinyl alcohol layer on said film base. This treatment makes itpossible to achieve a value for the surface tension of the filmstretched monoaxially which is in excess of that of the coating of thepolyvinyl alcohol and of the water-dispersible copolyester and which ispreferably equal to or in excess of 54 mN.m.

The aqueous solution of the polyvinyl alcohol and of thewater-dispersible copolyester used has generally a concentration of 1%to 20% in weight/weight and, preferably of 5% to 15% in weight 1 weight.Generally, in the first instance, a solution of polyvinyl alcohol isprepared. This solution is prepared carefully first at room temperatureby simply stirring, then by heating to a temperature not exceeding 95°C; after cooling, the solution is filtered. This solution must begel-ree. The absence of gel must be controlled by turbidity, dry mattercontent and refractive .index measurements. The water-dispersiblecopolyester is dissolved or dispersed in this solution of polyvinylalcohol.

The weight ratio of the polyvinyl alcohol to the water dispersiblecopolyester in said aqueous solution is preferably from 99/1 to 50/50and ranges preferably from 97/3 to 80/20 and even more preferably from95/5 and 85/15.

The polyvinyl alcohol used in the process according to the invention isa compound available commercially. It can be used such as or prepared inparticular by hydrolysis of vinyl carboxylates, more particularly vinylpolyacetates or their copolymers rich in vinyl acetate residues, such asthe copolymers of vinyl acetate and ethylene (or EVA). As was mentionedpreviously, the polyvinyl alcohol used includes at least 95% vinylalcohol residues (degree of hydrolysis equal to or in excess of 95%).Preferably, it includes at least 97% and even more preferably at least98% thereof.

The polyvinyl alcohol used for the preparation of the composite coatingof the polyvinyl alcohol and of the water-dispersible copolyesterexhibits a viscosity in a 4% aqueous solution and at 20° C., measured inan apparatus of the Brookfield type, which is equal to or in excess of 4centipoises (or 4 mPa.s), which corresponds, within experimental errors,to an average polymerisation degree in number equal to or in excess of350.

The choice of the concentration of the solution of the polyvinyl alcoholand of the water-dispersible copolyester, as well as the apparatus usedfor the coating, is in particular, dependent on the desired thicknessfor the final coating of the polyvinyl alcohol and of thewater-dispersible copolyester.

Coating operations are usually carried out with gravure rollersaccording to the reverse gravure technique. However, the process of theinvention is not limited to this embodiment.

In the case of an on-line coating, the polyester film base is, the moreoften, stretched longitudinally (i. e. in the machine direction) beforesaid coating by means of an aqueous solution of the polyvinyl alcoholand of the water-dispersible copolymer, without this version limitingthe invention.

This stretching can be carried out in one or in several steps, which isalso the case of the stretching which follows the coating.

The temperature at which the coated film is treated is preferablybetween 180° C. and 240° C. or, even more preferably, between 200° C.and 230 ° C.

The polyester film base can be prepared by extrusion of a polyester suchas defined above, including one or several inert fillers to provide, inparticular, sufficient slippage characteristics. It can also be preparedby co-extrusion, on the one hand of a polyester low in filler and, onthe other hand, of a filled polyester.

In a second version, the polyester film base comprises a filled roughlayer providing said back face and a less filled layer, providing saidfront face, which will be coated with the solution of the polyvinylalcohol and of the water-dispersible copolyester.

The filled polyester layer generally exhibits an average roughness Rzwhich is equal to or in excess of 0.15 micrometres and, preferably equalto or in excess of 0.30 micrometres, whereas the less filled or nonfilled front layer exhibits a total roughness equal to or lesser than0.40 micrometres and preferably, equal to or lesser than 0.25micrometres ; as indicated previously, the face of the film base, whichwill be coated with the solution of the polyvinyl alcohol an of thewater-dispersible copolyester, carries preferably, on the average, notmore than 20 peaks having a height equal to or in excess of 1 micrometreand not more than 100 peaks having a height comprised between 0.4 and 1micrometre per square millimeter.

The deposition of the metal coating is carried out by any known means.Preferably, use is made of a vacuum metallization technique.

The composite films of the invention exhibit excellent properties ofresistance against mechanical aggression, such as in particular,crumpling or folding. This means that the barrier properties alreadydiscussed are little or not diminished after such a mechanicalaggression.

The following examples illustrate the invention.

EXAMPLES GENERAL EXPERIMENTAL PROCEDURE

A composite film is prepared from polyethylene terephthalate (PET)through the co-extrusion on the one hand of a PET containing 0.7% silicaobtained by precipitation of gels and exhibiting an average diameter of3.3 micrometres (size measurements carried out with a laser sizer<<SYMPATEC>> of the <<Helos>> type) which is the more filled film (backpart of the composite film support) and, on the other hand, of a PETwith a lower filler content (0.025% of the same silica) of which theouter face will receive a coating of a polyvinyl alcohol(PVA)/water-dispersible copolyester (COPO).

The polyester film has a thickness of 12 micrometres in the differentexamples or comparative tests and an average roughness Rz of 0.45 on theback face. It has a less filled layer (receiving the coating of the PVAand of the water-dispersible copolymer) which has a thickness of 11.2micrometres.

The co-extruded film is first stretched longitudinally with stretchratio of 3.4;

then the same is subjected to a corona treatment which adjusts itsurface tension to 58 mN.m.

The face of the film with the less filled layer is then coated by meansof a gravure coating system using an engraved roller. The speed of thefilm in the coating operation is of 200 m/min. The on-line coating (OLC)is carried out using an aqueous solution of PVA and of COPO having a 10%concentration, carefully prepared and devoid of gel. The PVA used has98-99% vinyl alcohol residues and a,viscosity of 5.5 mPa.s (measured at20° C. in a 4% aqueous solution by means of a viscosimeter of theBrookfield LV type). The COPO used is a copolyester of ethylene anddiethylene having an average number molecular weight of about 33 000 andincluding terephthalate and isophthalate residues in a ratio of about80/20 in weight, with 14% weight of sodium 5-oxysulfonyl-isophthalate inthe total copolyester.

The coated film is then subjected to a transverse stretching with astretch ratio of 4 and is then subjected to a thermal treatment at atemperature of 225° C. The thickness of the PVA/COPO layer is given foreach example and for the Comparative tests.

In the examples or in the comparative tests describing films includingtwo coatings, the coatings are applied on the bi-stretched film(longitudinal stretching with a stretch ratio of 3.4, followed by atransverse stretching with a stretch ratio of 4), in a separate coatingoperation (SCO).

On the different films dried at 200° C., measurements are made of thepermeability to oxygen at 23° C. with 50% relative humidity (O2P) incm³/m²/24 h) (measurements carried out using an <<OXTRAN>> apparatus,type 300H from the firm MODERN CONTROL INC.) and an adhesion test iscarried out on the coating using the <<adhesive tape>> test, where anadhesive tape (3M N° 250) is applied on the coated film on which hadbeen applied beforehand two layers of ink (TERCHEM 81), first a blue oneand then a white one. Thereafter, a 4 kg roller is run over the sample,five times both ways, the tape is then peeled off by pulling manuallyalong the axis of the sheet; the quality of the adhesion is evaluatedfrom the amount of ink peeled away with the adhesive tape (score on ascale from 0 to 10: a score of 8 to 10 indicates an excellent adhesionof the coating onto the polyester film).

Examples 1 and 2

These tests are carried out with a composite polyester film of which theface receiving the coating of PVA/COPO exhibits the following peakheight distribution, per mm²: 7 peaks of a height >1 μm, 49 peaks of aheight between 0.4 and 1 μm. The coating is carried out on-line (OLC)with aqueous solutions of PVA/COPO (the weight ratio PVA/COPO and thetotal concentration in the solution are given in table 1 hereafter).

Examples 3 and 4

The same composite polyester film was used in these examples 3 and 4. Inthese examples, after an on-line coating carried out with a solution ofPVA/COPO, a separate coating was carried out with an aqueous solution ofPVA only on the bi-stretched film, coated with PVA/COPO. Themeasurements are the same as in the other examples and the results aregiven in the table 1 hereafter.

Comparative Tests A, B and C

The same composite polyester film was used in these tests as in examples1 and 2. In these tests, an on-line coating operation was carried outwith an aqueous solution containing only COPO and then a separatecoating operation was carried out on the bi-stretched film with anaqueous solution of PVA. Thus, a polyester film base was obtainedcarrying two successive coatings, the first one consisting of awater-dispersible copolyester and the second one of a polyvinyl alcohol.The measurements were the same as those carried out in the otherexamples and the results are given in table 1 hereafter.

Comparative Test D

This test was done on the same composite film as in examples 1 and 2. Inthis test, an on-line coating was carried out with an aqueous solutioncontaining only PVA. The measurements were the same as in the examplesand the results are given in table 1 hereafter.

In table 1 hereafter, the following abbreviations are used, in additionto the abbreviations defined previously:

<<Ex>> stands for <<example>>

<<CT>> stands for <<comparative test>>

<<Ad. test>> stands for o adhesion test>>

<<Concsol w 1 w>> stands for (total concentration of the aqueoussolution in weight>>

<<Thick>> stands for <<thickness>>

TABLE 1 OLC SCO Con- Con- PVA/ csol Thick, csol Thick, Ad. Test COPO w/wμm PVA w/w μm O2P test Ex. 1 90/10 10% 0.2 none none none 1.0 1.0 Ex. 299/01 10% 0.2 none none none 1.0 8.5 Ex. 3 90/10 10% 0.2 present 2.5%2.5 0.6 9 Ex. 4 90/10 10% 0.2 present 10% 6 0.1 9 CT a 0/100 2.5% 0.02present 8% 0.41 15 2 CT b 0/100 5% 0.04 present 2% 0.07 14.6 5 CT c0/100 5% 0.04 present 9% 0.34 7.4 5 CT d 100/0 12% 0.1 present none —2.6 5

Example 5 and Comparative Test E

The same polyester composite film was used in these tests as in examples1 and 2. In these tests, an on-line coating was carried out using anaqueous solution of PVA/COPO (example 5) or an aqueous solution of PVAonly (comparative test e), with a total concentration of 12% in thesolution. The weight ratio of PVA 1 COPO was of 90/10 and the PVA had asaponification degree of 98%.

These films were metallized by deposition of an aluminum layer of 0.035μm.

The O2P of the metallized films was measured under the conditionsindicated for the preceding examples.

Each one of the two metallized films was laminated with a polyethylenefilm (of 50 μm) using a polyurethane adhesive and an adhesion test wascarried out.

This test consists in applying on the film a pulling force with anINSTRON 1122 apparatus, at the speed of 100 mm/min. The peeling strengththus measured is expressed in grams per 15 millimeters. A peelingstrength of 150 g/15 mm is considered as satisfactory for commerciallysold metallized films. The results of these tests are given in table 2hereafter.

TABLE 2 Test PVA/COPO O2P Peeling strength, g/15 mm Example 5 90/10 0.1220 Comparative example 100/0  0.2 105 e

Example 6 and Comparative Test F

In these tests, the same polyester composite film as in examples 1 and 2was used. The coating was carried out on-line with an aqueous solutionof PVA 1 COPO (example 6) or with an aqueous solution of PVA only(comparative test), with a total concentration of 12% in the solution.The weight ratio PVA/COPO was of 90/10 and the PVA had a saponificationdegree of 98%.

The O2P of these films was measured under the conditions defined for theprevious examples.

Each one of the two films was laminated on a polyethylene film (of 50μm) by means of a polyurethane adhesive and an adhesion test was carriedout as described in example 5 and in the comparative test e.

A peeling strength of 250 g/15 mm is considered as satisfactory forcommercially sold non metallized films. The results of these tests aregiven in table 3 hereafter.

TABLE 3 Test PVA/COPO O2P Peeling strength, g/15 mm Example 6 90/10 0.1320 Comparative test f 100/0  2.6 200

What is claimed is:
 1. A composite film stretched biaxially, and havingbarrier properties to gases, comprising a polyester film base of 5/m to50 μm, coated on one of its two faces with a layer including a polyvinylalcohol which has a number average degree of polymerization equal to orgreater than 350 and a degree of hydrolysis equal to or greater than 95%and a water-dispersible copolyester with sulphonyloxy residues, saidcomposite film exhibiting a permeability to oxygen measured at 23° C.under a relative humidity of 50% equal to or less than 5 cm³/m²/24 h. 2.A film according to claim 1, wherein the polyester is selected frompolyterephthalates or from polynaphthalene dicarboxylates of alkylenediols.
 3. A film according to claim 2, wherein the polyester is selectedfrom polyterephthalates of ethylene glycol or of 1,4-butanediol and thecopolyesters including at least 80% in moles of ethylene glycolterephthalate residues.
 4. A film according to one of claims 1 to 3,wherein the average roughness Rz of the film base is less than or equalto 0.40 μm on the face of the film carrying the layer of the polyvinylalcohol and of the water-dispersible copolyester and this face exhibits,on the average, not more than 20 peaks having a height equal to orgreater than 1 μm and not more than 150 peaks having a height between0.4 and 1 micrometre per square millimeter, when the thickness of thelayer including the polyvinyl alcohol and the water-soluble copolyesterwith sulphonyloxy residues is less than 0.6 μm.
 5. A film according toone of claims 1 to 3, wherein the face of the film base carrying thelayer of the polyvinyl alcohol and of the water-dispersible copolyesterhas no more than an average of 20 peaks having a height equal to orgreater than 1 micrometre and no more than 100 peaks having a heightbetween 0.4 and 1 micrometre, per square millimeter.
 6. A film accordingto one of claims 1 to 3 wherein the water-dispersible copolyester withsulphonyloxy residues combined with the polyvinyl alcohol in the coatingis selected from copolyesters derived from at least one aromaticdicarboxylic acid and from at least one aliphatic diol, and carrying aplurality of sulphonyloxy groups of the formula (I): —(SO₃—)_(n)M  (I)in which n is equal to 1 or 2, M is a hydrogen atom, an alkali metal, anearth-alkali metal, a cationic ammonium or a cationic quaternaryammonium.
 7. A film according to claim 6, wherein the aromaticdicarboxylic acid used for preparing the water-dispersible copolyesteris selected from terephthalic acid, isophthalic acid, orthophthalicacid, naphthalene 1,4-dicarboxylic acid and their mixtures.
 8. A filmaccording to claim 7, wherein the aromatic dicarboxylic acid is selectedfrom mixtures of terephthalic acid with one or more other aromaticdicarboxylic acids.
 9. A film according to claim 8, wherein the aromaticdicarboxylic acid is selected from mixtures of terephthalic acid withisophthalic acid.
 10. A film according to claim 8, wherein the amount ofterephthalic acid expressed in moles is in the range from 20 to 99% ofthe total number of moles of non sulphonated diacids and in that thewater-dispersible copolyester includes sodium5-oxysulphonyl-isophthalate residues.
 11. A film according to claim 10,wherein the amount of terephthalic acid expressed in moles is in therange from 30 to 95%.
 12. A film according to claim 6, wherein the diolentering into the composition of the water-dispersible copolyester withsulphonyloxy groups, is selected from ethylene glycol, 1,4-butanediol,1,5-butanediol, 1,3-propanediol, 1,2-propanediol,2,2-dimethyl-1,3pronanediol (or neopentylglycol), 1,5-pentanediol,1,6-hexanediol, diethylene glycol, triethylene glycol, tetraethyleneglycol and cyclohexane dimethanol.
 13. A film according to claim 12,wherein the diol entering into the composition of the water-dispersiblecopolyester with sulphonyloxy groups, is selected from ethylene glycoland its oligomers.
 14. A film according to claim 6, wherein in thewater-dispersible copolyester, the dicarboxylic acid residues withsulphonyloxy groups amount to 5-30 moles for 100 moles of thedicarboxylic acid residues.
 15. A film according to claim 14, wherein inthe water-dispersible copolyester, the dicarboxylic acid residues withsulphonyloxy groups amount to 8-15 moles for 100 moles of thedicarboxylic acid residues.
 16. A film according to one of claims 1 to3, wherein the coating of the polyvinyl alcohol and of thewater-dispersible copolyester exhibits a weight ratio of the polyvinylalcohol to the water-dispersible copolyester of 99/1 to 50/50.
 17. Afilm according to claim 16, wherein the coating of the polyvinyl alcoholand of the water-dispersible copolyester exhibits a weight ratio of thepolyvinyl alcohol to the water-dispersible copolyester comprised between97/3 and 80/20.
 18. A film according to one of claims 1 to 3, whereinthe coating of the polyvinyl alcohol and of the water-dispersiblecopolyester has a thickness equal to or lesser than 3 μm.
 19. A filmaccording to one of claims 1 to 3, wherein said composite film furthercarries on the coating of the polyvinyl alcohol and of thewater-dispersible copolyester, a layer of polyvinyl alcohol.
 20. A filmaccording to claim 19, wherein said composite film further carries aprinted layer or a printed primer on one of its faces, or in that itfurther carries a metal layer on the additional layer of the polyvinylalcohol.
 21. A film according to claim 20, wherein the metal layer,which is deposited on the surface of the coating of the polyvinylalcohol and of the water-dispersible polyester not in contact with thepolyester film or which is deposited on the additional layer of thepolyvinyl alcohol, is made of a metal selected from aluminum, copper,chromium, nickel and silver.
 22. A film according to claim 21, whereinthe metal layer, which is deposited on the surface of the coating of thepolyvinyl alcohol and of the water-dispersible polyester not in contactwith the polyester film or which is deposited on the additional layer ofthe polyvinyl alcohol, is made of aluminum.
 23. A film according toclaim 21, wherein the thickness of the metal layer is in the range from0.01 μm to 0.6 μm.
 24. A film according to one of claims 1 to 3, whereinthe polyester base film is comprised of two layers exhibiting differentroughnesses Rz.
 25. A film according to claim 24, wherein the polyesterbase film comprised of two layers exhibiting different roughnesses isproduced with a technique of co-extrusion of 2 polyesters havingdifferent filler contents and selectively including different fillers.26. A film according to claim 25, wherein the polyester used is the samefor the two co-extruded layers and in that the layer receiving thepolyvinyl alcohol and water-dispersible copolyester layer is the lessfilled one.
 27. A process for the production of composite filmsaccording to one of claims 1 to 3, wherein the polyester base film iscoated, on one of its faces, by means of an aqueous solution including apolyvinyl alcohol exhibiting at least 95% of alcohol residues, saidpolyvinyl alcohol exhibiting in an aqueous 4% solution and at 20° C. aviscosity equal to or greater than 4 mPa.s and a water-dispersiblecopolyester and in that said coated film is treated thermally at atemperature equal to or greater than 170° C.
 28. A process according toclaim 27, wherein the face of the polyester film carrying the coating ofthe polyvinyl alcohol and the water-dispersible copolyester exhibits, anaverage roughness Rz less than or equal to 0.40 μm and does not carry onthe average more than 20 peaks having a height equal to or greater than1 micrometre and not more than 150 peaks having a height between 0.4 and1 micrometre per square millimeter, when the desired thickness of thecoating including the polyvinyl alcohol and the water-dispersiblecopolyester is less than 0.6 micrometres.
 29. A process according toclaim 27, wherein the coating of the polyester film base with an aqueoussolution of the polyvinyl alcohol and of the water-dispersiblecopolyester is carried out on line or as a separate operation.
 30. Aprocess according to claim 29, wherein the coating is carried outon-line.
 31. A process according to claim 27, wherein the aqueoussolution of the polyvinyl alcohol and of the water-dispersiblecopolyester is used at a concentration in the range from 1% to 20% inweight.
 32. A process according to claim 31, wherein the aqueoussolution of the polyvinyl alcohol and of the water-dispersiblecopolyester is used at a concentration in the range from 5% to 15% inweight.
 33. A process according to claim 27, wherein the weight ratio ofthe polyvinyl alcohol to the water dispersible copolyester in theaqueous solution is between 99/1 and 50/50.
 34. A process according toclaim 33, wherein the weight ratio of the polyvinyl alcohol to thewater-dispersible copolyester in the aqueous solution is between 97/3and 80/20.
 35. A process according to claim 27, wherein the polyvinylalcohol used includes at least 97% of vinyl alcohol residues.
 36. Aprocess according to claim 35, wherein the polyvinyl alcohol usedincludes at least 98% of vinyl alcohol residues.
 37. A process accordingto claim 27, wherein the temperature at which the coated film istreated, is between 180° C. and 240° C.
 38. A process according to claim37, wherein the temperature at which the coated film is treated, isbetween 200° C. and 230° C.
 39. A film according to one of claims 1 or 3wherein said composite film further carries a printed layer or a printedprimer on one of its faces, or in that it further carries a metal layeron the coating of the polyvinyl alcohol and of the water-dispersiblecopolyester.